1. Field of the Invention
This invention relates to a titanium article wherein improved corrosion resistance is achieved by the integral incorporation of a platinum group metal or alloy thereof directly or indirectly attached to a minor surface portion of said article.
2. Description of the Prior Art
Titanium, being a reactive metal, relies on the formation and stability of a surface oxide film for corrosion resistance. Under stable conditions, titanium can demonstrate remarkable corrosion resistant behavior. The reverse is also true, however, in that when the film is destabilized, extremely high corrosion rates may result. These conditions of instability are generally at the two extremes of the pH scale. Strongly acidic or alkaline solutions can create instability in the titanium oxide film.
Typically, in accordance with prior art practice, when using titanium in an area of uncertain oxide film stability, alloying elements have been added to the titanium to enhance the oxide film stability, thus increasing its effective usefulness at the pH extremes. This practice has proven most effective for the acid end of the pH scale. Several alloying elements have shown success in this regard, such as molybdenum, nickel, tantalum, niobium and precious metals. Of this group, the platinum group metals (PGM) offer far and away the most effective protection against corrosion. The platinum group metals are platinum, palladium, ruthenium, rhodium, iridium and osmium.
Stern et al. demonstrated this in 1959 in a paper titled. “The Influence of Noble Metal Alloy Additions on the Electrochemical and Corrosion Behavior of Titanium”. They found that as little as 0.15% Pd or Pt alloying additions greatly enhanced the stability of the oxide film on titanium, and thus the corrosion resistance, in hot reducing acid medium. Consequently, for many years the ASTM grade 7 titanium (Ti-0.15Pd) has been the standard material for use in severe corrosive conditions where unalloyed titanium is subject to corrosion. More recently, ASTM grade 16 (Ti-0.05Pd) and grade 26 (Ti-0.1 Ru) have been used as direct replacements for grade 7 because they are more economical and provide a level of corrosion resistance close to that of grade 7. Thus, they tend to be considered equivalent in less drastic corrosion applications.
The mechanism of protection afforded by platinum group metal additions to titanium is one of increased cathodic depolarization. The platinum group metals afford a much lower hydrogen overvoltage in acidic media, thereby increasing the kinetics of the cathodic portion of the electrochemical reaction. This increased kinetics translates to a change in the slope of the cathodic half reaction, leading to a more noble corrosion potential for the titanium. The active/passive anodic behavior of titanium allows for a small shift in corrosion potential (polarization) to effect a large change in the corrosion rate.
Prior work on the polarization behavior of titanium was performed by Stern and Wissenberg in 1959. In this work, titanium was galvanically coupled to other metals to observe the effects on the corrosion rate of titanium in sulfuric acid media. The researchers found that titanium coupled to platinum could exhibit a reduction in corrosion rate of up to 100 fold, as seen in the present invention. However, they were not directly attaching the platinum onto the titanium and thus did not realize the extraordinary benefits seen in the present invention. In order for Stern and Wissenberg to achieve a 100 fold reduction in corrosion, they found that the surface area of the platinum had to be 4 times the area of the titanium. Thus, their surface area ratio of Ti/PGM was ¼. In fact, at a 35/1 Ti/PGM surface area ratio, the authors saw no benefit from the platinum coupling whatsoever. This was clearly not advantageous in terms of cost and thus, it is presumed the researchers then pursued alloying as the means of enhanced environmental behavior, as disclosed in U.S. Pat. No. 3,063,835. In the present invention using a direct or indirect attachment or integral incorporation practice, the 100 fold corrosion rate reduction is observed at all Ti/PGM surface area ratios, even to ratios thousands of times larger than Stern and Wissenberg's.
Although the above-described prior art practices are effective for enhancing the corrosion resistance of titanium in severe corrosive conditions, alloying additions of precious metals and especially the platinum group metals are extremely expensive.